Automatic gain control system



April 3, 1934. 0 FARNHAM 1,953,553

AUTOMATlC GAIN CONTROL SYSTEM Filed April 26, 1932 Patented Ap 3, 1934 AUTOMATIC GAIN CONTROL SYSTEM Paul 0. Farnham, Boonton, N. J., assignor to Radio Frequency Laboratories, Incorporated, Boonton, N. 3., a corporation of New Jersey Application April 26, 1932, Serial No. 607,600

23 Claims.

This invention relates to automatic gain control systems for transmission circuits of the modulated carrier wave type, and particularly to systems in which the incorporation of the control system into any given design of transmission circuit does not necessitate the use of additional tubes.

An object of the invention is to provide an automatic gain control system in which a single tube functions both as an alternating current amplifier and as a diode rectifier. A further object is to provide a transmission circuit including a plurality of alternating current amplifiers, and in which one or more of the amplifiers has the additional function of a diode rectifier. A further object is to provide an automatic gain control system in which the adjustment of the output level by a control effective upon a portion of the system succeeding the gain control automatically effects a corresponding adjustment in the operation of the automatic gain control. More particularly, an object is to provide a transmission circuit in which the rectifier of an automatic gain control system is combined with a radio frequency amplifier stage, and in which a diode demodulator is combined with the first audio frequency amplifier.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. 1 is a fragmentary circuit diagram of a radio receiver embodying the invention, and V Fig. 2 is a diagram of the essential elements of the demodulator input circuits.

There is wide latitude in the design of the transmission circuit in which the novel automatic gain control system is incorporated and, in Fig. 1 the reference character 1 indicates generically any desired type of audion amplifier system interposed between an input or collector structure AG and a radio frequency amplifier 2. The amplifier 1 may include one or more tubes and, in the case of a superheterodyne, a frequency changer.

The tube 2 includes an auxiliary plate P1 in addition to the usual plate P, and the grid system may be of any appropriate design but preferably, as shown, includes a control grid G1, a screen grid G2 and a suppressor grid G3. So far as conearns the cathode K, the grids and the plate P,

the physical construction may conform to the usual pentode designs. The auxiliary plate P1 may be of comparatively small size, and cooperates with the cathode K to function as the rectifier of the automatic gain control system.

(ill. 250-20) The amplified radio voltage developed across the LC circuit by the amplifier 1 is impressed between the control grid G1 and the cathode K of the amplifier tube 2, the lower potential terminal of the circuit preferably being connected to a tap -l which is adjustable along the cathode bias resistance 5 to control the bias voltage E1 on the control grid. The plate circuit includes the primary 6 of a transformer having two secondary windings 7, 8 whose lower potential terminals are connected by an audio frequency bypass condenser C1 and which are tuned to the radio frequency by shunt condensersl and 02, respectively. When the tube 2 is the last intermediate frequency amplifier of a superheterodyne, the condensers C, 02 and 9 are adjustable for alinement but are not adjusted during the operation of the receiver.

The high (radio frequency) potential terminal of the winding 7 is connected to the auxiliary or diode rectifier plate P1 and the low potential terminal is connected to ground through a resistance R1 across which a direct current voltage is developed by rectification between elements K and P1 of the tube 2. The direct current potential on the plate P1 is more negative than that of cathode K by the potential drop Eiacross the bias resistor 4, and therefore no rectification takes place until the peak radio voltage developed between cathode K and plate P1 exceeds the bias voltage E4. The direct current potential across R1 is employed as an automatic gain control voltage, the lead 10 by which the bias voltage is transferred to the tube or tubes of amplifier 1 being connected to the junction of the winding 7 and' the resistance R1.

The succeeding tube 11 of the transmission system is preferably of the same design as tube 2, but the auxiliary plate is identified by reference character Pa for convenience in distinguishing between the diode rectifiers of the tubes 2 and 11. The circuits of the tubes 11 are so arranged that rectification precedes the amplifier action, 1. e., the modulated signal is first rectified and the resulting audio frequency voltage is then amplified by the tube. The diode rectifier circuit includes, in addition to the tuned input circuit, 8C2, the audio frequency resistance R2 that is connected between the input circuit and the tube cathode K. Any desired portion of the audio frequency voltage developed across the resistance R2 may be impressed upon the control grid 611 of tube 11 through the circuit which in cludes the adjustable tap 12, a radio frequency filter consisting of resistance 13 and condensers 14, 15, and a blocking condenser 16.

An appropriate grid bias for audio amplification is impressed on the control grid from a source indicated as -C, and through a grid resistor 17. The amplified audio output voltage developed in the plate circuit is passed to the succeeding audio amplifier 18 which works into a loud speaker or other appropriate reproducer or load 19.

The operation of the system is as follows: So long as the received signals are of such relatively low magnitude that the amplified radio voltage across the Winding 7, and impressed between cathode K and auxiliary plate P1 of tube 2, is less than the delay bias voltage E4 on this diode rectifier, no rectification takes place and the amplifier l operates at its normal or maximum gain. The amplified radio voltage output of the tube 2 is also impressed on the input circuit of the demodulator or diode rectifier PaK of tube 11, and the audio frequency voltage output of that rectifier is amplified by tube 11 and passed to the succeeding load circuits.

When the peak radio voltage across the diode rectifier of tube 2 exceeds the delay bias E4, a gain control bias is developed across the resistance R1 and applied to the amplifier 1 to reduce the gain to maintain an approximately constant radio voltage level across the amplifier 2. The audio by-pass condenser C1 reduces the audio distortion which would otherwise result from the cutting of the modulation peaks by the gain control rectifier, since the audio component of the rectified voltage across the resistance R1 is thereby passed to the audio frequency resistance. R2.

Control of the audio output level at the reproducer 19 is effected by adjusting the tap 12 along the resistance R2 to determine that portion of the available demodulator output which is impressed upon the audio amplifier system. It is to be noted, however, that adjustment of the tap 12 has a control operation additional to that of the usual voltage divider. This will be apparent from a consideration of Fig. 2 which is an equivalent or simplified circuit diagram of the amplifier input circuits.

The tuned input circuit of the demodulator or diode rectifier K-Pa of tube 11 includes elements additional to the winding 8, the tuning condenser C2 and the effective resistance R of the circuit. The total capacity across the winding 8 includes the effective input capacity of the diode, and this capacity C3 is in series with the portion R2 of resistance R2 which isincluded between the winding 8 and the tap 12. As the magnitude of Ra is increased to lower the output level, the effective series resistance of the resonant circuit is increased and its eiiiciency is correspondingly decreased, thus reducing the audio frequency drop across the resistance R2. Further, when the circuits including windings 6 and 8 are coupled by substantially critical coupling, this increase in the effective resistance of the secondary circuit, to reduce the output level, results in a decrease in the effective resistance of the winding 6 circuit, thus increasing the radio voltage output of tube 2 for a given input voltage and thereby increas ing the automatic gain control voltage developed by tube 2. This increase in gain control voltage across R1 reduces the gain of the amplifier 1, and thereby produces a change in the automatic gain control system in the same sense as the manual change in the audio output system.

The rate at which the radio frequency gain is reduced with reductions in the audio frequency gain is determined by the ratios of the capacities C2 and C3, and may be controlled by appropriate design of the demodulator input circuit.

This reduction of the radio frequency gain with reductions in the audio frequency output is particularly desirable in a circuit in which the demodulator and first audio amplifier are combined in a single tube. If a high radio level were maintained at the diode elements of tube 11, leakage between the elements would prevent the audio output from falling to zero when the tap 12 is positioned at the lower potential end of resistance R2 to suppress all audio output.

In my copending application Ser. No. 536,936, filed Jan. 15, 1932, Automatic control systems, I have described and claimed a receiver of the automatic gain control type and having a multiple function stage which amplifies at radio frequency and then rectifies the amplified voltage to produce a gain control voltage, the amplifier and rectifier elements being housed in a single tube.

It will be apparent that the invention is not limited to the specific circuit herein illustrated and described. Modifications of the circuit without departure from the spirit of the invention herein described will occur to those familiar with the design and construction of alternating current transmission circuits, and such modifications are to be understood as included within the scope of the following claims.

I claim:

1. In a transmission system, the combination with a multistage radio frequency amplifier having a final stage working into a demodulator, of automatic gain control means including rectifier elements for controlling the gain of said amplifier, the rectifier elements of said gain control means being included in the tube of said final amplifier stage.

2. In a transmission system, a multistage radio frequency amplifier including a multiple-function stage having amplifier and rectifier elements housed within a single vacuum tube, circuits for applying theoutput of the rectifier to said amplifier as an automatic gain control voltage, and a diode demodulator and an audio frequency amplifier, said demodulator and the audio frequency amplifier into which said demodulator works comprising a single multiple element vacuum tube.

3. In a multistage transmission system, a double-purpose stage comprising a vacuum tube having an auxiliary anode additional to the normal tube elements required for amplification, said auxiliary anode cooperating with the tube cathode to constitute a diode rectifier, input and output circuit elements cooperating with said normal tube elements to form a radio frequency amplifier, a demodulator having an input circuit coupled to the said output circuit elements, an input circuit for said diode coupled to one of said output circuit elements, an output impedance for said rectifier and across which a direct current voltage is developed by the rectifier action, and a gain control circuit for automatically varying the gain of said transmission system to maintain a substantially constant voltage level at a predetermined point thereof, said gain control circuit including the said rectifier output impedance.

4. In a multistage system for the transmission of modulated signal waves, the combination with a single tube and circuit elements cooperating therewith to amplify a radio frequency voltage and to produce a direct current potential by rectification of the amplified radio frequency voltage developed by said tube, and means energized by said direct current potential for automatically controlling the gain of a signal frequency amplifier stage, of a demodulator, an audio frequency amplifier, and output level control means for impressing upon said audio frequency amplifier a desired portion of the audio frequency voltage developed by said demodulator.

5. The invention as set forth in claim 4, wherein said demodulator and audio frequency amplifier comprise a single vacuum tube and circuit elements cooperating therewith to complete a diode rectifier and an audio frequency amplifier.

6. The invention as set forth in claim 4, wherein said output level control means comprises resistive impedance in the input circuit and in the output circuit of said demodulator, and adjustable means for increasing the effective magnitude of the resistive impedance in said input circuit simultaneously with a movement of said adjustable means to reduce the output level, thereby reducing the efiiciency of the demodulator.

7. In apparatus for receiving signals of the modulated carrier wave type, the combination with a radio frequency amplifier, means for automatically controlling the gain of the amplifier in accordance with changes in the magnitude of received signals, a rectifier, an audio frequency load circuit working out of said rectifier, and adjustable means for varying the ratio of the audio frequency voltage level at the output of said load circuit to the radio frequency voltage impressed upon said rectifier and for simultaneously controlling the gain of said radio frequency amplifier.

8. The invention as set forth in claim 7, wherein said adjustable means varies both the audio frequency Voltage level and the amplifier gain in the same sense.

9. In a radio receiver, the combination with a radio frequency amplifier, a detector, an audio frequency load circuit, and automatic means for varying the amplifier gain to maintain the radio frequency voltage on the detector substantially independent of fluctuations in the received signal strength, of means adjustable in one sense to vary the transmission efficiency of said detector and audio frequency load circuit and to simultaneously vary the radio frequency voltage on said detector.

10. The invention as set forth in claim 9, wherein said adjustable means decreases the radio voltage on the detector when varied in the said sense to reduce the transmission of audio frequency currents.

11. In a radio receiver, the combination with a radio frequency amplifier, a detector having input and output circuits, said amplifier having an output circuit coupled to the detector input circuit, an audio frequency load circuit coupled to the detector output circuit, and means for automatically varying the amplifier gain in accordance with variations in the magnitude of received radio frequency voltages, of adjustable means in one of said detector circuits to vary the transmission efficiency of said detector and audio frequency load circuit, said adjustable means being operative through said coupled amplifier and demodulator circuits to vary the radio frequency voltage on said demodulator simultaneously with said variations in the transmission efficiency.

12. The invention as set forth in claim 11, wherein said amplifier output and demodulator circuits are coupled with substantially critical coupling.

13. In a radio receiver, the combination with a radio frequency amplifier and automatic control means for varying the gain thereof as a function of the strength of received radio frequency voltages, of a diode detector, a tuned input circuit for said detector and coupled to said amplifier, an audio output resistance for said diode detector, said input circuit and output resistance being serially connected between the elements of said detector, an audio frequency amplifier, a circuit connection between one input terminal of said audio amplifier and that terminal of said output resistance which is remote from said detector input circuit, an adjustable tap on said output resistance and connected to the other input terminal of said audio amplifier, and a radio frequency by-pass condenser connected between said tap and the terminal of said output resistance which is connected to said audio amplifier.

14. The invention as set forth in claim 13, wherein said detector input circuit is coupled to said radio frequency amplifier with substantially critical coupling.

15. The invention as set forth in claim 13, wherein said diode detector and audio frequency amplifier comprise a single vacuum tube and circuit elements cooperating therewith.

16. In the operation of a receiver of the type including a radio frequency amplifier having an automatic gain control, a detector, and control means for varying the audio output level, the method which comprises reducing the amplifier gain simultaneously with adjustment of said control means to reduce the audio output level.

17. In the operation of a receiver of the type including a radio frequency amplifier having an automatic gain control, a detector, an audio output device, and means for varying the transmission efficiency between the amplifier output and the audio output device, the method of regulating the audio level at the output device which comprises varying the gain of the amplifier simultaneously with variations in the said transmission efiiciency.

18. In a transmission system, a radio frequency amplifier including a final stage comprising an amplifier tube having a radio frequency output circuit, a diode detector and a diode rectifier connected in parallel across said output circuit, and means returning to said amplifier as a gain control voltage the direct current potential developed by said diode rectifier, one of said diodes comprising elements housed within said amplifier tube.

19. The invention as claimed in claim 18, wherein the said diode elements housed within said tube are the elements of said diode rectifier.

20. The invention as claimed in claim 18, wherein the said diode elements housed within said tube are the elements of said diode rectifier, in combination with means impre sing on said diode rectifier a bias voltage which renders the same inoperative when the radio output voltage of said amplifier falls below a predetermined critical value.

21. In a transmission system, a multiple function stage comprising a tube housing a cathode cooperating with a control grid and plate to form an amplifier and an anode located outside the electron stream controlled by said control grid, a radio input circuit connected between said control grid and cathode, an output circuit in which an amplified radio voltage is developed by the action of said amplifier elements, means impressing said amplified voltage between said anode and cathode, a resistance across which a direct current voltage is developed by the diode rectifier action of said anode and cathode, a detector, and means impressing on said detector the amplified radio voltage developed in said output circuit.

22.'The invention as claimed in claim 21 in 5 combination with means establishing on said control grid a negative potential appropriate for radio frequency amplification by the amplifier elements of said tube, and means establishing on said anode a direct current potential that is negative with respect to said cathode. 

